1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly, to an NAND type non-volatile ferroelectric memory cell and a non-volatile ferroelectric memory of the same.
2. Background of the Related Art
FIG. 1 illustrates a system of a related art NAND type DRAM.
Referring to FIG. 1, the NAND type DRAM cell is provided with a plurality of NMOS transistors T1, T2, T3, - - - connected in series, each having a gate connected to a wordline WL1, WL2, WL3, WL4, - - - . There is a bitline B/L formed in a direction crossing the wordlines, and there are ferroelectric capacitors C1, C2, C3, - - - each having one electrode connected to a drain terminal N1, N2, N3, - - - of the transistor and the other electrode connected to a plateline P/L, on which a constant voltage of ½ voltage is provided. Upon enabling the wordlines WL1, WL2, WL3, - - - in succession, NMOS transistors connected thereto are enabled, to provide data stored in the ferroelectric capacitors to the bitline. The data provided to the bitline is amplified at a sense amplifier (not shown), and re-stored in the ferroelectric capacitor.
The ferroelectric memory, i.e., an FRAM (Ferroelectric Random Access Memory), having in general a data processing speed similar to a DRAM (Dynamic Random Access Memory) used widely as a semiconductor memory and being capable of conserving data even if the power is turned off, is paid attention as a next generation memory. The FRAM, a memory having a structure similar to the DRAM, is provided with a capacitor of a ferroelectric material for utilizing a high residual polarization of the ferroelectric material. The residual polarization permits the conservation of a data even after removal of an electric field.
FIG. 2 illustrates a characteristic curve of a hysteresis loop of a general ferroelectric material.
Referring to FIG. 2, it can be known that a polarization induced by an electric field is, not erased, but, certain amount (‘d’ and ‘a’ states) of which is remained, even if the electric field is removed owing to existence of the residual polarization (or spontaneous polarization). The ‘d’ and ‘a’ states are corresponded to ‘1’ and ‘0’ respectively in application to memories.
A related art non-volatile ferroelectric memory will be explained with reference to the attached drawings. FIG. 3 illustrates a system of unit cell of the related art non-volatile ferroelectric memory.
Referring to FIG. 3, the system of unit cell of the related art non-volatile ferroelectric memory is provided with a bitline B/L formed in one direction, a wordline W/L formed in perpendicular to the bitline, a plateline P/L formed spaced from the wordline in a direction identical to the wordline, a transistor T1 having a gate connected to the wordline and a source connected to the bitline, and a ferroelectric capacitor FC1 having a first terminal connected to a drain of the transistor T1 and a second terminal connected to the plateline P/L.
A circuit for driving the related art ferroelectric memory will be explained. FIGS. 4A and 4B illustrate a circuit for driving the related art nonvolatile ferroelectric memory.
The circuit for driving the related art ferroelectric memory of an 1T/1C (one transistor and one ferroelectric capacitor) structure is provided with a reference voltage generator 1 for generating a reference voltage, a reference voltage stabilizer 2 having a plurality of transistors Q1˜Q4 and a capacitor C1 for stabilizing reference voltages on adjacent two bitlines, a first reference voltage storage 3 having a plurality of transistors Q6–Q7, and capacitors C2–C3 for respectively having logical values of “1” and “0” stored in adjacent bitlines, a first equalizer 4 having a transistor Q5 for equalizing adjacent two bitlines, a first main cell array 5 connected to wordlines and platelines different from each other for storing data, a first sense amplifier 6 having a plurality of transistors Q10˜Q15, a P-sense amplifier PSA and the like for sensing data in cells selected by the wordline among the plurality of cells in the first main cell array 5, a second main cell array 7 connected to wordlines and platelines different from one another for storage of data, a second reference voltage storage 8 having a plurality of transistors Q28˜Q29 and capacitors C9˜C10 for having reference voltages with logical values of “1” and “0” stored in adjacent bitlines, and a second sense amplifier 9 having a plurality of transistors Q16˜Q25, N-sense amplifier NSA and the like for sensing and forwarding a data in the second main array 7.
The data input/output operation of the related art ferroelectric memory will be explained. FIG. 5 illustrates a timing diagram of a write mode operation of the related art ferroelectric memory, and FIG. 6 illustrates a timing diagram of a read mode operation of the related art ferroelectric memory.
In writing, when an external chip enable signal CSB pad is enabled from ‘high’ to ‘low’ and a write enable signal WEBpad is applied from ‘high’ to ‘low’ on the same time, the write mode is started. When address decoding is started in the write mode, a pulse applied to a pertinent wordline is transited from ‘low’ to ‘high’ to select a cell. Thus, during a period the wordline is held ‘high’, a pertinent plateline has a ‘high’ signal applied thereto for one period and a ‘low’ signal applied thereto for the other period in succession. And, in order to write a logical value ‘1’ or ‘0’ on the selected cell, a ‘high’ or ‘low’ signal synchronized to the write enable signal WEBpad is applied to a pertinent bitline. That is, if a ‘high’ signal is applied to the bitline and a signal applied to the plateline is ‘low’ in a period in which a signal applied to the wordline is ‘high’, a logical value ‘1’ is written on the ferroelectric capacitor. And, if a ‘low’ signal is applied to the bitline and a signal applied to the plateline is ‘high’, a logical value ‘0’ is written on the ferroelectric capacitor.
The operation for reading the data stored in the cell by the aforementioned write mode operation will be explained.
If the chip enable signal CSBpad is enabled from ‘high’ to ‘low’ externally, all bitlines are equalized to a ‘low’ voltage before a pertinent wordline is selected. That is, referring to FIGS. 3A and 3B, if a ‘high’ signal is applied to the equalizer 4 and a ‘high’ signal is applied to the transistors Q18 and Q19, as the bitlines are grounded through the transistor Q19, the bitlines are equalized to a low voltage Vss. And, the transistors Q5, Q18, and Q19 are turned off, to disable the bitlines, an address is decoded, and the decoded address causes a ‘low’ signal on a pertinent wordline to transit to a ‘high’ signal, to select a pertinent cell. A ‘high’ signal is applied to the plateline of the selected cell, to break a data corresponding to a logical value ‘1’ stored in the ferroelectric memory. If a logical value ‘0’ is in storage in the ferroelectric memory, a data corresponding to the logical value ‘0’ is not broken. The data not broken and the data broken provide values different from each other according to the aforementioned hysteresis loop, so that the sense amplifier senses a logical value ‘1’ or ‘0’. The case of the data broken is a case when the value is changed from ‘d’ to ‘f’ in the hysteresis loop of FIG. 2, and the case of the data not broken is a case when the value is changed from ‘a’ to ‘f’ in the hysteresis loop of FIG. 2. Therefore, if the sense amplifier is enabled after a certain time period is passed, in the case of the data broken, a logical value ‘1’ is provided as amplified, and in the case of the data not broken, a logical value ‘0’ is provided. After the sense amplifier provides data thus, since an original data should be restored, the plateline is disabled from ‘high’ to ‘low’ in a state a ‘high’ signal is applied to a pertinent wordline.
The aforementioned related art ferroelectric memory has the following problem.
The frequent use of one reference cell required for reading main cells a few hundred times more than the main cells under a state the ferroelectric properties are not assured perfectly, that requires the reference cell much more operation than the main cells, causes a rapid degradation of the reference cell, resulting in a voltage instability, device operation characteristics deterioration, and short lifetime.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.